1. Field of the Invention
The present invention relates to an electro-photographic photoreceptor which, for example, can be adapted to copiers and printers, and to an image-forming apparatus using the same.
2. Description of the Related Art
An electrophotographic technology invented by C. F. Carlson affords an instantaneous and highly preservative image of high quality. This technology, accordingly, has widely been employed in the field of copiers and, recently, spread out into the field of a variety of printers and facsimile apparatus. The image-forming process in the electrophotographic technology basically comprises a step of electrically charging a photoreceptor uniformly, a step of forming an electrostatic latent image by exposure, a step of developing the electrostatic latent image with toner, a step of transfering the toner image onto a sheet of paper and a step of fixing the transferred toner image. Alternatively, the transfer of the toner image onto a sheet of paper may be achieved through an intermediate transfer.
As for the photoreceptor which works as the core of the electrophotographic technology, in addition to conventional inorganic photoconductive materials such as selenium, arsenic-selenium alloy, cadmium sulfide and zinc oxide, a photoreceptor made of organic photoconductive materials has recently been developed, which is advantageous in the view that it is nonpolluting and easily produced because film formation is easy.
A so-called laminated photoreceptor, which is made by laminating a charge generation layer and a charge transport layer, has become the mainstream in current photoreceptors and has been produced on a large scale since it is highly sensitive, the scope of selection of photoreceptive materials is so broad as to give a highly safe photoreceptor, and productivity of the coating is so high as to make the cost relatively advantageous.
Recently, on the other hand, in order to obtain an image of higher quality, and store and edit freely an input image, the digitization of image formation rapidly progresses. Heretofore, adaptation of the digital image-forming apparatus has been limited to output devices, i.e., laser printers of word processors or personal computers, LED (light emitting diode) printers and some of color laser copiers. The digitalization, however, has been extended to the field of copiers in which analogue image formation has mainly been employed until now.
It is required that the performance of the photoreceptor of the image-forming apparatus responding to such digitalization satisfies the following requirements roughly classified into three.
The first requirement is that it is sensitive to light of long wavelength. In forming a digital image, a digital electric signal has to be converted into a light signal in order to directly use information from a computer. On the other hand, when information is inputted from an original document, the information is read as light information, then once converted into digital electric signals, and converted again into light signals. In any case, the information is inputted as light signals into the photoreceptor, and the input of the digital electric signals as light signals may be achieved by means of laser or LED light. Input light which is now most frequently employed is a near infrared radiation having an oscillating wavelength of 780 nm or long wavelength light having an oscillating wavelength of 650 nm. The first requirement for the photoreceptor used in the digital image formation is that the photoreceptor has to be sensitive to such long wavelength light, and a wide variety of materials have been examined until now. In particular, phthalocyanine compounds have widely been examined and practically used since they can be synthesized in a relatively simple manner and most of them are sensitive to long wavelength light.
The second requirement is that there is no increase of residual electric potential. In forming a digital image, the image is formed by making toner adhere on a light-irradiated portion in order to effectively utilize the light or enhance resolution, that is, a reversal development process is employed in many cases. In the reversal development process, an unexposed part (electric potential of the dark part) becomes a white background and the exposed part (electric potential of the light part) becomes a black ground (line of image). In the reversal development process, accordingly, fog phenomenon, which yields black spots on the white background, never occurs even though the electric potential of the light part increases as in the normal development process, but when the electric potential of dark part decreases the fog phenomenon occurs. Therefore, a scorotron charger may often be used as a charger to keep a given electric potential of dark part at all times.
Moreover, a part at which the electrical charge potential decreases appears as a dark spot in the image. This is likely to be caused by injection of the electric charge from the conductive support, and therefore, in order to prevent the injection of the electric charge from the conductive support, such provision is made for the photoreceptor that an intermediate layer is interposed between the photoreceptive layer and the conductive support. However, since charge-blocking property of a simple resin film provided as an intermediate layer is so large as to cause an increase in residual electric potential. Therefore, a coating film is used as an intermediate layer in which conductive particles such as titanium oxide are dispersed into the resin. When the content of the resin in the intermediate layer is increased, the blocking property is enhanced to prevent occurrence of dark spots on the image, but the residual electric potential rises. On the other hand, when the content of titanium oxide is increased, the coating film tends to be uneven to yield dark spots on the image. Japanese Unexamined Patent Publication JP-A 6-202366 (1994) proposes various types of titanium oxides and combination rates of the resin to titanium oxide in the intermediate layer.
The third requirement is that there is no occurrence of moire, i.e., striped pattern of light and shade caused by light interference in an image. In drawing a digital image on the photoreceptor, a laser light is employed. A coherent light such as laser light is apt to cause interference. In the photoreceptor, incident ray interferes with the light reflected from the conductive support to yield moire on the image. In such a case, it has been proposed that the surface of support makes rough or an opaque intermediate layer is provided between them.
The photoreceptor satisfying the above three requirements and responding to digitalization, however, has the following disadvantage. Such a disadvantage is that, though the photoreceptor involving a phthalocyanine compound as an charge generation material is highly sensitive at the long wavelength, the charged voltage is low in the first rotation and is stabilized just after the second rotation. This phenomenon relates to the standing time after the image formation process such as electrical charging and photo-exposure. When the standing time is longer, then the charged voltage at the next first rotation is apt to be low. This phenomenon is considered to relate to the fact that the dark charge is generated by the phthalocyanine compound during standing to accumulate in the charge generation layer.
In an image-forming apparatus which is equipped with a photoreceptor using a phthalocyanine compound as a charge generation material and in which a reversal development process is employed, it is a disadvantage that the charged voltage at the first rotation is so low as to yield a fog phenomenon quite often as mentioned above and necessitates preliminary rotation as warming up once or more. Accordingly, much time is required from the start of operation of the image-forming apparatus to the actual image formation.
Heretofore, the time required for the preliminary rotation was not serious problem because a data transfer speed from a computer to a printer was lower and the image-processing time in a digital copier was longer. In recent years, however, the function of microcomputers is greatly improved and the data transfer and image-processing can be achieved quite rapidly. Therefore, it is required that the image formation could be achieved at the first rotation of the photoreceptor to speed up copying or printing of the first sheet of paper.
When the photoreceptor involving a phthalocyanine compound as a charge generation material is used in the image formation process at the first rotation, however, a change of contrast sometimes occurs due to low charged voltage at the first rotation as mentioned above and in a serious case a fog phenomenon is generated. As a result, the preliminary rotation must be made to slow down the rate of copying or printing.
In Japanese Unexamined Patent Publication JP-A 9-127711 (1997), the photoreceptive layer contains phthalocyanine and a specific azo pigment, but this is not sufficient for inhibiting decrease in the charged voltage at the first rotation.
On the other hand, Japanese Unexamined Patent Publication JP-A 11-52601 (1999) discloses a technique in which the intermediate layer contains a metal complex or metal salt of aromatic carboxylic acid and a binder resin to inhibit an effect of moisture change.